1. Field of the Invention
The invention relates to a digital data transmission system providing multipoint connections.
Multipoint connections ae defined in "Contribution No. 281" of Studies Committee No. VII of the International Telegraph and Telephone Consultative Committee (C.C.I.T.T.), Sixth Plenary Assembly, Geneva, 1976. In this kind of connection a number of data transceivers or "data terminal equipment" (D.T.E.) are connected to a single data transmission link. One transceiver is the main transceiver and the others are secondary. All of the messages transmitted by the main transceiver reach the secondary transceivers while the secondary transceivers transmit singly and one at a time to the main transceiver.
2. Description of the Prior Art
The fundamental parameters of a multiplexing scheme for the international interface between synchronous data networks is defined in "Recommendation X50" of Internation Telegraph and Telephone Consultative Committee (C.C.I.T.T.), Fifth Plenary Assembly, Geneva, December 1972, Green Book, Volume VIII, Data Transmission, Published by the International Telecommunications Union, pages 201 through 203. According to this recommendation the multiplex aggregate by rate of 64 kbits/s is standardized for international links and framing information for the channels is contained within the 64 kbits/s capability. The signal elements of each individual channel are assembled in octets. The status bits are included in the octets and are allocated to the last bit position in the octets. An octet interleaved structure is used. A distributed framing pattern is used with the framing bits occupying the first bit position in consecutive octets.
It results that, in each octet, the first bit position is reserved for multiplex framing, the second to seventh bit positions are occupied by channel information bits and the eighth bit position is reserved for the status bit. The addition of a framing bit and a status bit to six information or signaling bits results in an increase of one-third in the binary flow rate of information. Consequently, the binry flow rates on the component low rate data are respectively 12.8, 6.4, 3.2 and 0.8 kbits/s when the binary flow rates for the user are 9.6, 4.8, 2.4 and 0.6 kbits/s respectively.
A 64 kbits/s channel be formed by multiplexing five 12.8 kbits/s channels, ten 6.4 kbits/s channels, twenty 3.2 kbits/s channels or eighty or eighty 0.8 kbits/s channels. The octets belonging to the channels at these respective rates will repeat every fifth octet (5 octet frame), every tenth octet (10 octet frame), every twentieth octet (20 octet frame) and every eightieth octet (80 octet frame). In these various frames the lower common multiple of the octet numbers is 80 which is chosen for defining the multiframe.
The 80 octets of a multiframe are referenced by their first bits which form a multiframing sequence. This sequence of bits is pseudo-random and has a period equal to 80 which is obtained in a known manner by a shift register in which a certain number of intermediate stages and the final stage are looped to the input stage via an exclusive OR-gate.
Time division multiplex digital data switching networks are known in the prior art and are, for example, disclosed in U.S. Pat. No. 3,987,251, issued Oct. 19, 1976. In this Patent intermediate rate synchronous digital data channels are made up by time multiplexing a plurality of component digital data having different low rates which are sub-multiples of said intermediate rate. To fix one's ideas, the intermediate rate is 64 kbits/s and the low rates are 0.6, 2.4, 4.8 and 9.6 kbits/s. Then a plurality of 256 incoming 64 kbits/s channels, in which the component low rate data are multiplexed with a period depending on their rates, are in turn multiplexed to form a high rate multiplex highway comprising eight 2.048 kbits/s multiplex junctions on which the octets appear in parallel. Thus each octet in the high rate multiplex highway is defined by an address having a first part of 8 bits (1out of 256) denoting the channel number in the highway and a second part of 7 bits (1out of 80) denoting the octet number in the multiframe.
Such a switching network switches a parallel octet from the incoming high rate highway to a parallel octet location in the outgoing high rate highway. More specifically, those octets of the incoming highway which belong to a low rate component channel assigned to a calling subscriber are transferred to the locations of parallel octets in the outgoing highway which belong to a component channel of the same low rate assigned to the called subscriber. After the switching operation has been carried out, new framing bits forming a pseudo-random sequence are disposed in the first bit locations of the outgoing octets. The switching operation depends on signaling information borne by the signaling octents which, as stated, differ from the information octets since they contain a status bit having a different binary value.
In the invention the digital switching network of the prior art is used like a distribution frame, i.e., it permanently connects an incoming low rate component channel to an outgoing low rate component channel. It is not necessary here to repeat the description of the structure of a digital switching network given in the aforementioned United States Patent but sufficient to state that the switching network can be considered as set in the position in which it is placed when a subscriber transmitting digital data has called a called party.